Might be too early. Did you check with your dr before introducing them early?
I think nappy rash is more common once LOs are on solids as the ph of the poop is different and has more irritants in especially if it is passing through only partially digested.
Is it green? That could mean she has a virus. I agree with Martina Rae, check with the dr if you are worried.
When DS started solids at 6mo he got a sore bottom and I got some cream from the dr, I think it was called Metformin...? It was great, much more heavy duty than sudocrem we normally use.